It is known that a magneto-Archimedes effect caused by a magnetic field having a magnetic field gradient (which is referred to as “gradient magnetic field” hereinafter) is used to float or levitate material objects in a supporting liquid. For example, Patent Document 1 and Non-Patent Document 1 described below disclose a method for separating a mixture of a plurality of types of plastic particles by particle type using the magneto-Archimedes effect.